A conventional glow plug used for ignition assistance for internal combustion engines includes a heater in which a resistor formed of a conductive ceramic is disposed inside a substrate formed of an insulating ceramic. The resistor includes two rod-shaped lead portions, an approximately U-shaped joint portion that connects one end of one of the lead portion to one end of the other lead portion, and electrode portions disposed so as to protrude from the lead portions toward the outer circumferential surface of the substrate. The resistor generates heat when current is supplied to the resistor through the electrode portions. The resistor and substrate used for the heater are produced from materials each containing a ceramic and a binder (such as a resin). For example, as described in Japanese Patent Application Laid-Open (kokai) No. 2007-240080, a green intermediate molded product that later becomes the resistor in a subsequent step is formed by injection molding of a molding material containing a ceramic and a binder, and the intermediate molded product is subjected to debindering and firing, whereby the resistor is produced.